“She saw.” The “she” is Judah. The people of Judah saw Israel’s sin and destruction. Although the Massoretic Hebrew text reads “I saw,” that seems to be a copyist’s error; one Hebrew manuscript, along with the Septuagint and Aramaic texts read “she,” which fits the context.
“sent her away.” The country of Israel was conquered by the Assyrians and the people of Israel were deported to Assyria (2 Kings 17:5). However, when the Assyrian army tried to conquer the country of Judah, the godly Hezekiah was king and they were defeated by a miracle (2 Kings 19:35-36).
“divorce.” Israel sinned so egregiously that God divorced her (cp. Isa. 50:1).